A device-cooling base oil includes 30 mass% of at least one of an oleyl ester (e.g., oleate and oleyl alcohol ester) and oleyl ether. The oleyl ester and the oleyl ether each have 23 or more of a total number of a terminal methyl group, a methylene group and an ether group in a main chain and 1 or less of a total number of a methyl branch and an ethyl branch. The base oil has a kinematic viscosity at 40 degrees C in a range of 4 mm 2 /s to 30 mm 2 /s. A device-cooling oil provided by blending the base oil is excellent in electrical insulation properties and thermal conductivity, and thus is favorably usable for cooling a motor, a battery, an inverter, an engine, an electric cell or the like in an electric vehicle, a hybrid vehicle or the like.